Astrocytes are increasingly understood to be active players in the etiology and treatment of Major Depressive Disorders (MDD, or depression) and offer a new biological area for development of novel antidepressant drugs. The goal of this project is to understand the molecular mechanisms of action of antidepressant drugs on astrocytes. The knowledge gained will create opportunities to identify novel targets for discovery of new antidepressant drugs, and biomarkers that predict or stratify drug response. We have developed an innovative, high throughput, in vitro platform for genomewide association studies (IV-GWAS) of drug response. In this project we will use a genetically diverse population of astrocytes differentiated from mouse embryonic stem cells. IV- GWAS vastly increases the number of genetic screens over conventional live animal or human GWAS. Further, the in vitro environment can be tightly controlled, which increases statistical power and enables more precise experimental designs. We will identify genes that modify phenotypes relevant to depression in astrocytes treated with antidepressant drugs. We will test almost all of the antidepressant drugs in current use, a set of 40 compounds. In Phase I, we will pre-screen the compounds and phenotypes to identify those exhibiting the largest genetic variability (Heritability). In Phase II we will conduct IV-GWAS using ~300 cell lines to identify genes that regulate or predict variable response to drug treatment. We will validate these findings in both mouse and human astrocytes.